The present invention relates to a fuel injector having an integrated spark plug.
European Published Patent Application No. 0 661 446 concerns a fuel injector having an integrated spark plug. The fuel injector have an integrated spark plug is used to inject fuel directly into the combustion chamber of internal combustion engine and to ignite the fuel that is injected into the combustion chamber. Installation space at the cylinder head of the internal combustion engine can be economized through the compact integration of a spark plug in a fuel injector. The known fuel injector having an integrated spark plug includes a valve body, which, together with a valve-closure member actuatable by a valve needle, forms a sealing seat. Contiguous to the sealing seat is a spray orifice, which discharges at a valve-body end face facing the combustion chamber. The valve body is insulated by a ceramic insulating body from a housing body that is able to be screwed into the cylinder head of the internal combustion engine. Disposed on the housing body is a ground electrode for producing a counter voltage to the high voltage being applied to the valve body. When the valve body is loaded with sufficiently high voltage, a spark arcing-over takes place between the valve body and the ground electrode connected to the housing body.
It is believed that one problem with such a fuel injector having an integrated spark plug, however, is that the position of the spark arc-over is not defined with respect to the fuel jet spray-discharged from the spray orifice, since the spark arc-over can take place at virtually any point in the lateral region of a valve-body projection. The so-called root of the fuel jet spray-discharged from the spray orifice cannot be ignited with the level of certainty required for this known type of construction. However, a reliable and precisely timed fuel-jet ignition is absolutely essential for reducing pollutant emissions. In addition, coking and sooting can constantly progress at the fuel-jet discharge orifice, affecting the spray-discharged jet form.
In contrast, it is belieed that one advantage of the fuel injector having the integrated spark plug of an exemplary embodiment of the present invention is that the spark arc over position is able to be reproducibly and unambiguously defined with respect to the spray-orifice position. It is also believed that this ensures a reliable ignition of the spray-discharged fuel jet. The spark arc-over position and, thus, the ignition point can be placed in the region of the spray-discharged fuel jet having the least significant, cyclical jet fluctuations. Therefore, the instant of fuel-jet ignition exhibits extremely small fluctuations from injection cycle to injection cycle. Positioning the spark arc-over, (that is, the ignition point in the vicinity of the spray orifice) counteracts any sooting and coking effects and, thus, acts in opposition to any changes in the jet geometry resulting therefrom.
The edge for defining the spark arc-over position can either be provided at the valve-body end face or at the ignition electrodes. The edge at the valve-body end face can be formed by a protuberance or indentation. In this context, it is advantageous that the valve body have a rounded flank region for specifically targeting the air flow to the ignition point. One or a plurality of pin-shaped ignition electrodes can be secured to the housing body, inclined at a predefined angle toward the valve-body end face. In this context, one edge of the ignition electrodes constitutes the point having the smallest distance to the valve-body end face and, thus, defines the ignition point. When the edge defining the ignition point is formed at the valve-body end face, a simple wire spanning the valve-body end face can also be used as an ignition electrode, which is an especially cost-effective design.
The ignition electrode can quite advantageously have a ring-shaped design, including an opening for the fuel jet spray-discharged from the spray orifice. In this context, the edge defining the ignition point is formed at the opening of the annular ignition electrode. To avoid hindering the fuel jet, it is advantageous for the opening of the annular ignition electrode to widen conically in the spray-discharge direction of the fuel jet, with the opening angle of the ignition electrode being advantageously adapted to the opening angle of the fuel jet. Designing the mount fixture for the ignition electrode with radially distributed bar-type projections and with pins, arranged radially with respect to the projections, ensures an adequate, radial, oncoming combustion-air flow and reinforces reliable fuel-jet ignition.